Why does copy paste keyframes paste keyframes additional to those I selected?
Selected channels
As the tooltip suggests, when you paste from the buffer, the keyframes are pasted into the selected channels.
If no channel is selected, the keyframes are pasted into all the editable channels.

Duplicate
In my opinion, this is the easiest way.
You can duplicate the keyframes (Shift + D) and move them by entering the number of frames or by moving the mouse.
Include only selected objects
Alternatively, you can include only the channels related to the selected object.

or
Protect channels
You can protect the other channels from editing:

Select the channel
Press Tab (locked / unlocked)